This study aims to assess the immune response and safety of GSK's candidate chickenpox and marketed MMR vaccines when given to children 12 to 15 months of age via a muscle injection. It compares the GSK vaccines to Merck's chickenpox vaccine, administered just under the skin. Additionally, the study will evaluate the immune response and safety of giving the GSK vaccines along with other childhood vaccines through a muscle injection.

Inclusion Criteria:

• Participant's parent(s)/ Legally acceptable representatives (LAR[s]), who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of the eDiaries, return for follow-up visits).

• Written or witnessed/thumb printed informed consent obtained from the participant's parent(s)/LAR(s) prior to performance of any study-specific procedure.

• Healthy participants as established by medical history and clinical examination before entering into the study.

• A male or female between, and including, 12 to 15 months of age (i.e., from the day of 1-year birthday until the day before 16 months of age) at the time of the administration of study interventions.

• Only for children in countries where PCV is recommended at 12 to 15 months of age as per national immunization schedule and provided as part of the study interventions:

  • Participant who previously received the primary series of PCV in the first year of life with last dose at least 60 days prior to the administration of study intervention.

Exclusion Criteria:

• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study interventions including hypersensitivity to neomycin or gelatin.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination (no laboratory testing required).
• Hypersensitivity to latex.
• Major congenital defects, as assessed by the investigator.
• Recurrent history of uncontrolled neurological disorders or seizures.
• History of measles, mumps, rubella, or varicella disease.
• Active untreated tuberculosis.
• Participants with bleeding disorders (e.g., thrombocytopenia or any coagulation disorder).
• Condition that in the judgment of the investigator would make intramuscular injection unsafe.
• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the study.

Prior/Concomitant therapy

• Use of any investigational or non-registered product (drug, vaccine or invasive medical device) other than the study interventions during the period beginning 30 days before the dose of study interventions administration (Day -29 to Day 1), or their planned use during the study period.

• Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune modifying treatments at any time up to the end of the study.

  - Up to 90 days prior to the study intervention administration:

• For corticosteroids, this will mean prednisone equivalent >=0.5 mg/kg/day with maximum of 20 mg/day for pediatric participants. Inhaled and topical steroids are allowed.

• Administration of immunoglobulins and/or any blood products or plasma derivatives.

  - Up to 180 days prior to study interventions administration: long acting immune-modifying drugs including among others immunotherapy (e.g., tumor necrosis factor-inhibitors), monoclonal antibodies (except the ones not interfering with the immune response to the study vaccines, e.g., nirsevimab), antitumoral medication.

• Previous vaccination against measles, mumps, and rubella.

• Previous vaccination against varicella virus.

• Previous vaccination against hepatitis A virus.

• Only for children in countries where PCV is recommended at 12 to 15 months of age as per national immunization schedule and provided as part of the study interventions, participant who previously received a booster dose of any PCV.

Prior/Concurrent clinical study experience

• Concurrently participating in another clinical study, at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational intervention (drug/invasive medical device).

Other exclusion criteria

• Any study personnel or their immediate dependents, family, or household members.
• Child in care.
• Participants with the following high-risk individuals in their household: • Immunocompromised individuals.
• Pregnant women without documented history of varicella.
• Newborn infants of mothers without documented history of varicella.
• Newborn infants born <28 weeks of gestation

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

This is an Open Label study.